The invention concerns a monitoring method and a device for carrying out the monitoring method. The monitoring method and the device serve for monitoring the thickness of continuously conveyed, flat objects, in particular of printed products. With the inventive method it is possible to e.g. monitor the number of pages of printed products, to distinguish between products with correct and products with incorrect numbers of pages and to initiate corresponding further treatment.
In order to detect individual pages being missing or superfluous in printed products possibly having very different thicknesses by means of thickness measurement, a measuring accuracy of less than one tenth of a millimeter is necessary whereby the total thickness of the products may vary from below one millimeter to a few centimeters.
Known systems for measuring the thickness of continuously conveyed printed products often work with a contact-sensor which presses a printed product to be measured against the support on which the printed product lies for being conveyed, whereby the distance between contact-sensor and support is measured directly or indirectly. In order to achieve a measuring accuracy as mentioned above using contact-sensors and distance measurement the participating mechanical parts must be produced and assembled to a very high degree of precision. This is especially important if the contact-sensor belongs to a measuring system and the support to a conveying system and if, due to high conveying speeds and conveying performance, a large number of contactsensors and an even larger number of supports is used.
In the publication EP-651231 (F362) such a system is described. The measuring system substantially consists of a rotating disc with several pairs of contact-sensors which e.g. co-operate with the saddle-shaped supports of a gathering drum (conveying system). The disc and the gathering drum are arranged and synchronized relative to each other such that each support and the printed product conveyed on it is contacted by one pair of contact-sensors. The difference between the measuring positions (support without and with product) of the two contact-sensors corresponds to the thickness of the contacted product. These measurements are very sensitively dependent not only on the relative arrangement of the contact-sensors of each pair and on the relative arrangement of the two sensor arrangements for measuring the positions of the contact-sensors, but also on the arrangement of the supports. In order to achieve a sufficiently long measuring time, the contact-sensors must be movable relative to the disc and their additional movement superimposed on the circular movement of the disc must be controlled by corresponding control means. For elimination the influence of many mechanical tolerances on such a measurement, calibration, i.e. measurement without printed products on the supports, is necessary for each combination of contact-sensor pair and support in addition to the support measurements carried out with each thickness measurement and each thickness measurement is to be corrected according to the calibration. For keeping the number of calibration measurements within sensible limits the number of supports is to be an integer multiple of the number of contact-sensor pairs.
It is an object of the invention to provide a method for monitoring the thickness of flat objects, in particular of printed products, which flat objects are conveyed continuously by one conveying means each and to create a device for carrying out the method. The inventive method and the inventive device are to be independent of the conveying device to a considerably higher degree than this is the case in known devices. This means that the inventive thickness monitoring is not to enforce substantial mechanical constraints and increased accuracy on the conveying system. In spite of the necessary high resolution (paper thickness in the region of 0.1 mm, thickness of product up to several cm), the device is to be realizable using simple mechanical means and production and assembly tolerances easily achieved. The device is to be robust and to need little adjustment, i.e. it is to have characteristics which guarantee operation without problems in the dusty climate of printing works.